1. Field
The subject technology relates generally to signal generation, and more specifically to fractional-N synthesized chirp generators.
2. Background
Modern radar systems often employ so-called chirped-FM waveforms in which the frequency of the transmitted signal is varied linearly between an upper and lower frequency at a rapid, and often times variable, rate of change with time. In such applications, it is important that the rate of change of the frequency be constant during an interval of up-chirp and/or down-chirp as inaccuracies in this parameter degrade the accuracy performance of the radar system as well as its sensitivity.
One approach to generating a chirped-FM waveform involves applying a voltage ramp to a low phase noise voltage controlled oscillator (VCO). This approach is limited by inherent nonlinearities in the frequency versus control voltage tuning curve of the VCO. Correction for this error term, while possible, is problematic. Another significant problem, and one that is less easily dealt with, is the frequency noise associated with the VCO. At low frequency offsets, the VCO's frequency noise power spectral density is typically quite poor, and this often degrades system performance. This problem is particularly acute in monolithic implementations where limitations on the achievable quality factor of on-chip passive structures results in much poorer VCO frequency and phase noise than that associated with off-chip circuits.